Python Programming:
An Introduction to
Computer Science
Chapter 7
Objectives
To understand the programming pattern
simple decision and its implementation
using a Python if statement.
To understand the programming pattern
two-way decision and its implementation
Objectives (cont.)
To understand the programming pattern
multi-way decision and its
implementation using a Python
if-elif-else statement.
To understand the idea of exception
handling and be able to write simple exception handling code that catches
Objectives (cont.)
To understand the concept of Boolean
expressions and the bool data type
(requires Python 2.3 and newer)
To be able to read, write, and implement
algorithms that employ decision
structures, including those that employ sequences of decisions and nested
Simple Decisions
So far, we’ve viewed programs as
sequences of instructions that are followed one after the other.
While this is a fundamental
programming concept, it is not sufficient in itself to solve every problem. We need to be able to alter the sequential flow of a program to suit a particular situation.
Simple Decisions
Control structures allow us to alter this
sequential program flow.
In this chapter, we’ll learn about decision
structures, which are statements that allow a program to execute different sequences of instructions for different cases, allowing the program to “choose”
Example:
Temperature Warnings
Let’s return to our Celsius to Fahrenheit
temperature conversion program from Chapter 2.
# convert.py
# A program to convert Celsius temps to Fahrenheit # by: Susan Computewell
def main():
celsius = input("What is the Celsius temperature? ") fahrenheit = 9.0 / 5.0 * celsius + 32
Example:
Temperature Warnings
Let’s say we want to modify that
program to print a warning when the weather is extreme.
Any temperature over 90 degrees
Fahrenheit and lower than 30 degrees Fahrenheit will cause a hot and cold weather warning, respectively.
Example:
Temperature Warnings
Input the temperature in degrees Celsius
(call it celsius)
Calculate fahrenheit as 9/5 celsius + 32
Output fahrenheit
If fahrenheit > 90
print a heat warning
If fahrenheit > 30
Example:
Temperature Warnings
This new algorithm has two decisions at
the end. The indentation indicates that a step should be performed only if the
condition listed in the previous line is true.
Example:
Example:
Temperature Warnings
# convert2.py
# A program to convert Celsius temps to Fahrenheit. # This version issues heat and cold warnings.
def main():
celsius = input("What is the Celsius temperature? ") fahrenheit = 9.0 / 5.0 * celsius + 32
print "The temperature is", fahrenheit, "degrees fahrenheit." if fahrenheit >= 90:
print "It's really hot out there, be careful!" if fahrenheit <= 30:
Example:
Temperature Warnings
The Python if statement is used to
implement the decision.
if <condition>:
<body>
The body is a sequence of one or more
statements indented under the if
Example:
Temperature Warnings
The semantics of the if should be clear.
First, the condition in the heading is evaluated.
If the condition is true, the sequence of statements
in the body is executed, and then control passes to the next statement in the program.
If the condition is false, the statements in the body
are skipped, and control passes to the next statement in the program.
Example:
Example:
Temperature Warnings
The body of the if either executes or
not depending on the condition. In any case, control then passes to the next
statement after the if.
Forming Simple Conditions
What does a condition look like?
At this point, let’s use simple
comparisons.
<expr> <relop> <expr>
Forming Simple Conditions
Greater than >
>
Greater than or equal to ≥
>=
Equal to =
==
Less than or equal to ≤ <= Less than < < Meaning Mathematics Python
Forming Simple Conditions
Notice the use of == for equality. Since
Python uses = to indicate assignment, a
different symbol is required for the concept of equality.
A common mistake is using = in
Forming Simple Conditions
Conditions may compare either numbers
or strings.
When comparing strings, the ordering is
lexigraphic, meaning that the strings are sorted based on the underlying ASCII
codes. Because of this, all upper-case letters come before lower-case letters.
Forming Simple Conditions
Conditions are based on Boolean expressions,
named for the English mathematician George Boole.
When a Boolean expression is evaluated, it
produces either a value of true (meaning the condition holds), or it produces false (it does not hold).
Forming Simple Conditions
Boolean conditions are of type bool and the
Boolean values of true and false are
represented by the literals True and False. >>> 3 < 4 True >>> 3 * 4 < 3 + 4 False >>> "hello" == "hello" True
Two-Way Decisions
Let’s look at the quadratic program as we left
it.
# quadratic.py
# A program that computes the real roots of a quadratic equation. # Illustrates use of the math library.
# Note: This program crashes if the equation has no real roots.
import math # Makes the math library available.
def main():
print "This program finds the real solutions to a quadratic" print
a, b, c = input("Please enter the coefficients (a, b, c): ")
discRoot = math.sqrt(b * b - 4 * a * c) root1 = (-b + discRoot) / (2 * a)
Two-Way Decisions
As the comment implies, when
b2-4ac < 0, the program tries to take the square
root of a negative number, and then crashes.
This program finds the real solutions to a quadratic Please enter the coefficients (a, b, c): 1,1,2
Traceback (most recent call last):
File "C:\Documents and Settings\Terry\My Documents\Teaching\W04\CS 120\Textbook\code\chapter3\quadratic.py", line 21, in main()
File "C:\Documents and Settings\Terry\My Documents\Teaching\W04\CS 120\Textbook\code\chapter3\quadratic.py", line 14, in main
Two-Way Decisions
We can check for this situation. Here’s our first attempt.
# quadratic2.py
# A program that computes the real roots of a quadratic equation. # Bad version using a simple if to avoid program crash
import math def main():
print "This program finds the real solutions to a quadratic\n" a, b, c = input("Please enter the coefficients (a, b, c): ") discrim = b * b - 4 * a * c
if discrim >= 0:
Two-Way Decisions
We first calculate the discriminant
(b2-4ac) and then check to make sure it’s
nonnegative. If it is, the program
proceeds and we calculate the roots.
Look carefully at the program. What’s
wrong with it? Hint: What happens when there are no real roots?
Two-Way Decisions
This program finds the real solutions to a
quadratic
Please enter the coefficients (a, b, c): 1,1,1 >>>
This is almost worse than the version
that crashes, because we don’t know what went wrong!
Two-Way Decisions
We could add another if to the end:
if discrim < 0:
print "The equation has no real roots!"
This works, but feels wrong. We have
two decisions, with mutually exclusive
outcomes (if discrim >= 0 then
Two-Way Decisions
In Python, a two-way decision can be
implemented by attaching an else
clause onto an if clause.
This is called an if-else statement:
if <condition>: <statements> else:
Two-Way Decisions
When Python first encounters this structure, it
first evaluates the condition. If the condition is true, the statements under the if are
executed.
If the condition is false, the statements under
the else are executed.
In either case, the statements following the
Two-Way Decisions
# quadratic3.py
# A program that computes the real roots of a quadratic equation. # Illustrates use of a two-way decision
import math def main():
print "This program finds the real solutions to a quadratic\n" a, b, c = input("Please enter the coefficients (a, b, c): ") discrim = b * b - 4 * a * c
if discrim < 0:
print "\nThe equation has no real roots!" else:
Two-Way Decisions
>>>
This program finds the real solutions to a quadratic Please enter the coefficients (a, b, c): 1,1,2
The equation has no real roots! >>>
This program finds the real solutions to a quadratic Please enter the coefficients (a, b, c): 2, 5, 2
